config.coffee contains configuration of your app. You can set plugins /
languages that would be used here.

app/assets contains images / static files. Contents of the directory would
be copied to build/ without change.
Other app/ directories could contain files that would be compiled. Languages,
that compile to JS (coffeescript, roy etc.) or js files and located in app are
automatically wrapped in module closure so they can be loaded by
require('module/location').